In operations such as synthetic plastic molding processes, and wire and cable reclamation where metal is separated from insulation, particularly where thermoplastics are used, scrap material is recycled by first cutting it into small pieces and then feeding it into extruders for further molding operations. To prepare the scrap material for such recycling, it is commonly fed through machines called granulators which cut the material into small pieces are of particulate size. Such machines are generally provided with a plurality of bed knives usually two, positioned around the cutting circle of a rotary cutting member which is generally plural bladed. The material to be cut up is fed into the granulator where it is cut by the cooperative action of the rotating fly knives on the rotor and the bed knives.
During the operation of such granulators when operating on thermoplastic material some plugging or blinding of the discharge screen frequently occurs. This plugging then leads to a decrease in the through-put or output of the granulator. In turn, a build-up of the material occurs in the cutting chamber resulting in an increase in temperature within the cutting chamber. Such plugging may eventually cause melting of the material being cut up. This results in stoppage of the machine and subsequent downtime for cleaning and in the more extreme situations, can even produce a fire hazard. The problem is more severe when the granulator is operated with relatively small size holes in the screen or when material with a low melting temperature is being processed, or when both effects are present in combination.
Against this background the present invention admirably provides for injecting pressurized fluid such as air into the cutting chamber of the granulator and in a direction at the rotor and generally perpendicular to the holes in the discharge screen. The rotation of the rotor interrupts the air blast causing an intermediate scrubbing of the screen to minimize dust build-up and agitate the material lying on the top of the screen. This tends to suspend or stratify the material in the air in and above the cutting chamber. Further, the introduction of this pressurized air improves the cooling of the cutting chamber and cutting knives by injecting additional cooling air and by allowing more efficient air flow due to reduced screen plugging and, in turn, cooling by the conventional air conveying discharge system. By preventing a heavy build-up of material directly on top of the screen and causing agitation within the chamber which serves to keep the material in the air stream within the hopper, the granulator is rendered more tolerant to occasional overfeeding. Plugging of the screen is essentially precluded and melting of the thermoplastic material within the cutting chamber is virtually eliminated. Thus, the overall efficiency of the granulator is markedly improved with the employment of the present invention.